This invention relates to high refractive index optical glasses having a low specific gravity for utilization as both distant vision and near vision spectacle lenses having refractive indexes nd=1.79-1.81, dispersions vd=31-29 and specific gravities s=3.4-3.7.
The first lightweight high refractive index glasses utilized as spectacle lenses became known in 1973. These glasses had a refractive index nd=1.70, a dispersion vd=30 and a specific gravity s=3.0.
The object of the present invention is to produce high refractive index glasses which allow an optician to produce spectacle lenses of high dioptric values having thinner edge thicknesses (negative lenses) and/or smaller center thicknesses (positive lenses) than those previously used. The shallow curvature of such lenses increases the optically effective field of vision and improves the cosmetic effects of the spectacle in which they are used. The specific gravity for high refractive index glasses must be maintained as low as possible if the glasses are to benefit the spectacle-wearer, e.g., where a spectacle glass has a refractive index nd=1.79 and a specific gravity of approximately 3.3 then a 46 mm diameter lens will be no heavier than a lens made of a glass having a lower refractive index nd=1.70, a dispersion vd=30 and a specific gravity s=3.0. Where the glass has a refractive index nd=1.79 and a specific gravity of 3.7, then the weight of a 46 mm diameter lens increases by approximately 10-14 percent (approximately 2-3 g) according to the dioptric value thereof.
Accordingly, the object of the invention is more particularly to produce optical glasses having the following properties:
(a) nd=1.79-1.81 PA1 (b) vd=31-29 PA1 (c) s=3.4-3.7 PA1 (a) they must have high crystallization stability to allow continuous production processes; PA1 (b) they must have good chemical resistance; PA1 (c) they must have chemical hardenability to withstand the falling ball tests of ASTM and DIN 4646; and PA1 (d) in order to use the glass not only for distant vision spectacle lenses but also for near vision spectacle lenses they must be capable of fusing with conventional, distant vision commercial crown glasses (nd=1.523) and light, higher refractive index glasses (nd=1.70) so that the expansion coefficient must consequently lie between 93 and 99.times.10.sup.-7 /.degree.C. in the range of 20.degree.-300.degree. C. and the viscosity related to temperature must be adjusted within the melting range to the viscosity of normal commercial distant vision glasses.
The optical glasses must also fulfill the following important conditions in order to be suitable for use as spectacle lenses:
Known optical glasses having an nd=1.79-1.81 do not fulfill the aforementioned requirements, as illustrated by the following examples:
______________________________________ .alpha. .times. 10.sup.7 /.degree. C. Type nd vd s (20.degree.-300.degree. C.) ______________________________________ Heavy Flint Glasses SF 6 1.8051 25.43 5.18 90 SF 11 1.7847 25.76 4.74 69 LaF and LaSF Glasses LaF 9 1.795 28.38 4.96 81 LaSF 8 1.80741 31.61 4.87 70 LaSF N 3 1.8080 40.75 4.68 72 LaSF 11 1.80166 44.26 4.62 70 LaSF 30 1.80200 46.47 4.90 71 ______________________________________
Other optical glasses are disclosed by German Pat. No. 1,260,712 having refractive indexes nd=1.78-1.81 with specific gravities .ltoreq.4.0 which, as a result of their strong tendency to crystallize, are not suitable for continuous mass production purposes (e.g., glasses for spectacle lenses and large volume crucible melting). The crystallization tendency is determined by the magnitude of the crystallization growth speed and the crystallization temperature range in relation to the viscosity.
Such known glasses having a refractive index nd&gt;1.75 contain 14-46 percent by weight of alkaline earth metals. Their ZnO content is 10-20 percent by weight, their TiO.sub.2 content is 10-25 percent by weight and their SiO.sub.2 content is 12-20 percent by weight. The alkali metal oxide content for glasses of nd&gt;1.79 must not exceed 3 percent by weight and such glasses are not chemically hardenable.
The cooling of these glass melts, according to the above noted German patent description, must be undertaken "very rapidly, in order to avoid the yellow-brown coloration". The melts must be cast with a viscosity of &lt;100 poises since the strong tendency to crystallize renders continuous and automatic production impossible. The high ZnO content of 10-20 percent by weight in conjunction with a 10-25 percent content by weight of TiO.sub.2 is unfavorable in this respect.